Internal combustion engines, whether spark- or compression-ignition types, are often employed for powering vehicles, either as a primary power source, or as part of a hybrid powertrain. Various systems have been specifically developed to treat exhaust gases of such engines.
In the case of compression-ignition, i.e., diesel, engines, an AT system frequently incorporates a diesel oxidation catalyst (DOC) and a selective catalytic reduction (SCR) catalyst. The primary use of the DOC is to reduce amount of hydrocarbons (HC) by oxidation and storing HC in the catalyst. In the presence of ammonia (NH3), the SCR catalyst is used to convert oxides of nitrogen (NOX) into diatomic nitrogen (N2) and water (H2O).
Generally, when the temperature of the exhaust gas reaches a predetermined value, the DOC lights-off, i.e., becomes activated and reaches operating efficiency, at elevated temperatures. Therefore, the DOC is sometimes mounted in close proximity, i.e., close-coupled, to the engine in order to reduce loss of thermal energy from the exhaust gas flow prior to the exhaust gas reaching the DOC.